1. Field of the Invention
The present invention relates to a lithographic printing plate precursor and lithographic printing process. More particularly, the invention relates to a lithographic printing plate precursor which gives a lithographic printing plate stably showing ink receptibility during printing and having excellent printing durability, and to a lithographic printing process.
2. Description of the Related Art
A lithographic printing plate generally has oleophilic image areas which receive an ink during printing and hydrophilic nonimage areas which receive a fountain solution. Lithographic printing is a process in which the surface of a lithographic printing plate is made to have a difference in ink adhesion by forming oleophilic image areas as ink-receiving areas and hydrophilic nonimage areas as fountain-solution-receiving areas (non-ink-receiving areas) based on the fact that water has the property of repelling oil-based inks, and an ink is adhered only to the image areas and then transferred to a material to be printed, e.g., paper, to conduct printing.
A lithographic printing plate precursor (PS plate) comprising a hydrophilic support and an oleophilic photosensitive resin layer (image-recording layer) formed thereon has hitherto been in wide use for producing such lithographic printing plate therefrom. Usually, a lithographic printing plate is produced from a lithographic printing plate precursor by a method which comprises exposing the precursor through an original, e.g., a lith film, and then dissolving and removing those unnecessary parts of the image-recording layer which become nonimage areas with an alkaline developing solution or organic solvent to thereby expose the corresponding surface parts of the hydrophilic support and form nonimage areas while leaving those parts of the image-recording layer which become image areas.
Such platemaking processes heretofore in use for producing a printing plate from a lithographic printing plate precursor necessitate a step in which unnecessary parts of the image-recording layer after exposure are dissolved and removed with a developing solution or the like. However, to eliminate or simplify such a wet treatment performed additionally is one of the subjects to be accomplished. In particular, the discard of waste liquids resulting from wet treatments has recently become a matter of considerable concern of the whole industrial world from the standpoint of care of the global environment and, hence, there is an increasingly growing desire for the accomplishment of that subject.
For this purpose, a technique called on-press development has been proposed as a simple platemaking method. In this technique, an image-recording layer is used whose unnecessary parts can be removed in an ordinary printing process. After exposure, the unnecessary parts of the image-recording layer are removed on a printing machine to obtain a lithographic printing plate.
Examples of the on-press development include: a method which uses a lithographic printing plate precursor having an image-recording layer capable of being dissolved or dispersed in a fountain solution or ink solvent or in a fountain solution/ink emulsion; a method in which an image-recording layer is mechanically removed by contact with rollers or the blanket of a printing machine; and a method in which the cohesive force of an image-recording layer or adhesion between the image-recording layer and the support is reduced by the penetration of a fountain solution, ink solvent, or the like and, thereafter, the image-recording layer is mechanically removed by contact with rollers or the blanket.
In the invention, the term “development step” means, unless otherwise indicated, a step in which an apparatus (usually, an automatic processor) other than printing machines is used to remove unexposed areas of the image-recording layer of a lithographic printing plate precursor by contact with a liquid (usually, an alkaline developing solution) to expose a surface of the hydrophilic support. Furthermore, the term “on-press development” herein means, unless otherwise indicated, a method and step in which a printing machine is used to remove unexposed areas of the image-recording layer of a lithographic printing plate precursor by contact with a liquid (usually, a printing ink and/or a fountain solution) to expose a surface of the hydrophilic support.
However, in the case of using the related-art image-recording layer in which images are recorded with ultraviolet or visible light, it has been necessary to employ a troublesome procedure in which the lithographic printing plate precursor which has been exposed is, for example, kept in a completely light-shielded state or under constant-temperature conditions until it is mounted on a printing machine because the image-recording layer does not fix even after the exposure.
On the other hand, digitization technology in which image information is electronically processed, accumulated, and outputted by a computer has recently come to spread extensively, and various new image output techniques suitable for such digitization technology have come to be practically used. Under these circumstances, attention is focused on a computer-to-plate technique in which a highly convergent radiation such as a laser light is caused to carry digitized image information and this light is used to scan and expose a lithographic printing plate precursor to directly produce a lithographic printing plate without via a lith film. Consequently, to obtain a lithographic printing plate precursor suitable for such a technique has become one of important technical subjects.
As described above, simplification of platemaking and use of a dry platemaking process involving no development step have recently come to be more strongly desired than before from the standpoints of care of the global environment and suitability for digitization.
High-output lasers such as a semiconductor laser emitting infrared rays having a wavelength of from 760 to 1,200 nm and a YAG laser have recently become available at low cost. Because of this, a process for lithographic printing plate production using any of these high-output lasers as a device for image recording is coming to be regarded as a promising process which employs scanning exposure and is easy to incorporate into the digitization technology.
In the conventional platemaking process, a photosensitive lithographic printing plate precursor is imagewise exposed at a low to medium illuminance to record an image based on an imagewise property change caused by a photochemical reaction in the image-recording layer. In contrast, in the above-described process using a high-output laser, a large quantity of light energy is applied to exposed areas in an extremely short time period to efficiently convert the light energy to heat energy and the image-recording layer is caused by this heat to thermally undergo a change such as a chemical change, phase change, or change in form or structure. This change is utilized for image recording. Consequently, although image information is inputted by means of light energy such as laser light, image recording is influenced not only by the light energy but also by the reaction caused by heat energy. Usually, the recording technique utilizing the heat generated by such high-power-density exposure is called heat mode recording, and the conversion of light energy into heat energy is called light/heat conversion.
Great merits of platemaking processes employing heat mode recording are that the image-recording layer is not sensitive to light on an ordinary illuminance level, such as indoor light, and that an operation for fixing the image recorded by high-illuminance exposure is not essential. Namely, there is no possibility that the lithographic printing plate precursor for use in heat mode recording might be influenced by indoor light before exposure, and it is not essential to conduct an operation for image fixing after exposure. Consequently, when there is a lithographic printing plate precursor capable of on-press development which employs an image-recording layer which is insolubilized or solubilized by exposure with, e.g., a high-output laser, then a printing system is possible in which the image is not influenced even when the image-recording layer after the exposure is exposed to indoor ambient light. Namely, it is expected that when heat mode recording is utilized, a lithographic printing plate precursor suitable for on-press development can be obtained. However, most of the related-art photosensitive recording materials practically useful as image-recording layers are sensitive to light in the visible region having wavelengths of 760 nm and shorter and, hence, image recording therein with an infrared laser is impossible. Because of this, there is a desire for a material capable of image recording with an infrared laser.
Under these circumstances, Japanese Patent No. 2938397, for example, describes a lithographic printing plate precursor comprising a hydrophilic support and, formed thereon, an image-forming layer comprising a hydrophilic binder and hydrophobic thermoplastic-polymer particles dispersed therein. There is a description in Japanese Patent No. 2938397 to the effect that this lithographic printing plate precursor can be used in the following manner. The precursor is exposed with an infrared laser to thermally bond the hydrophobic thermoplastic-resin particles to one another and thereby form an image. Thereafter, this precursor is attached to the cylinder of a printing machine and developed thereon with a fountain solution and/or an ink.
The technique described above in which an image is formed by the mere bonding of fine particles by thermal fusion attains satisfactory on-press developability. However, this technique has had a problem that image strength (adhesion to the support) is considerably low and printing durability is insufficient.
On the other hand, JP-A-2001-277740 and JP-A-2001-277742 describe a lithographic printing plate precursor which comprises a hydrophilic support and deposited thereon microcapsules containing a polymerizable compound.
Furthermore, JP-A-2002-287334 describes a lithographic printing plate precursor comprising a support and formed thereon a photosensitive layer (image-recording layer) comprising an infrared absorber, a radical polymerization initiator, and a polymerizable compound.
Those techniques utilizing a polymerization reaction have a feature in that image strength is relatively satisfactory because the image areas have a higher chemical-bond density than the image areas formed by the thermal fusion bonding of fine polymer particles.
However, the radical polymerization type photosensitive layer has a feature in that it is apt to be influenced by oxygen during image formation. It has hence been necessary to form a protective layer comprising a hydrophilic resin having high oxygen barrier properties, such as poly(vinyl alcohol). Such a protective layer has had drawbacks that the removal thereof in on-press development requires much times to incur a large paper loss and that in case where the protective layer is not completely removed and remains in a slight amount on the image areas, then the residue attracts a hydrophilic ingredient in the fountain solution to reduce ink receptibility. The reduced ink receptibility arouses troubles, for example, that the image areas during printing come to suffer ink adhesion failures, resulting in reduced printing durability. For overcoming these drawbacks, it is effective to add a hydrophilic compound to the protective layer and the image-recording layer and thereby enhance the penetration of a fountain solution in on-press development. However, this technique has had a problem that the addition of a hydrophilic compound simultaneously reduces the image strength and ink receptibility of the image areas and, hence, satisfactory printed matters are not obtained.